feat: add support for NOT EXISTS / NOT EXIST comparator

[azhar22k]

Closes #8

Description

CloudWatch filter expressions support checking whether a JSON attribute does not exist using the NOT EXISTS syntax (e.g. { $.SomeOtherObject NOT EXISTS }). This PR extends the Lark grammar and matcher parser to support this comparator.

Key Changes

• Grammar (grammar.lark):
  • Added comparison: ... | entity NOT_EXISTS -> not_exists
  • Added NOT_EXISTS: /(?i:NOT\s+EXISTS?)/ terminal matching both NOT EXISTS and NOT EXIST case-insensitively with flexible whitespace.
• Matcher (matcher.py):
  • Introduced MISSING sentinel object to distinguish between absent attributes and attributes explicitly set to null (None).
  • Added not_exists handler in FilterEvaluator to return True when the resolved entity is MISSING.
  • Refactored resolve_entity to safely traverse nested objects, dictionaries, and list indices without raising AttributeError or KeyError on missing intermediate paths.
  • Ensured comparisons on missing attributes safely return False.
  • Updated extract_boolean to support raw booleans as well as AST nodes.
• Tests:
  • Added parser test cases in tests/test_parser.py (including invalid syntax error cases).
  • Added matcher test cases in tests/test_matcher.py (missing fields, present fields, nested paths, list indices, null values, and combined logical expressions).

[pinzon]

LGTM. Thank you for contributing to this project